Aggrieved customer (BMW KEMPTON PARK)

I have recently purchased a 2nd hand vehicle from BMW Kempton Park. Its is a Focus ST. I had to wait a month to take delivery, due to the mechanics failing to find the source of a miss fire the car had. Through trial and error, they eventually claimed it was a coil pack failure. After I eventually took delivery, I drove the car for just 1 week, and it threw oil all over the engine bay and began smoking really badly. I took the car back to the dealership, only for them to tell me, it is not their responsibility. They insinuate, that I over rev the car, which I didn't. And in any event, this is a performance vehicle, so how can it break in just 1 week off buying it???? I believe I'm being treated extremely unfairly. I wonder if this will reach someone, that can actually assist me in a way forward.